The use of digital data communication has become widespread to the point of nearly being ubiquitous. For example, digital communications are routinely implemented in providing data communications in various systems, such as computer network systems (including personal area networks (PANs), local area networks (LANs), metropolitan area networks (MANs), wide area networks (WANs), the Internet, etc.) and communication network systems (including the public switched telephone network (PSTN), cellular networks, cable transmission systems, etc.).
Time synchronization may be employed in such networks for various uses, such as determining location, proximity, or speed. Moreover, a need for time synchronization may arise out of the reality that hardware clocks are not perfect, wherein variations in oscillators and/or drift in the clock signals may result in durations of time intervals of events not being the same between network nodes. Time synchronization can be an important issue in network communications, wherein uplink/downlink interference may be avoided with high synchronization accuracy. For example, a network may operate in synchronous mode (e.g., time-division duplexing (TDD) mode), in which transmissions by different network devices (e.g., base stations, access points, user equipment, mobile devices, etc., performing wireless communications utilizing various forms of receivers, transmitters, and/or transceivers) within the network are aligned in time and/or frequency. In such a synchronous network, synchronization of the network devices is of utmost importance because if a network device is not accurately synchronized to the rest of the network (e.g., synchronized to other network devices), its transmissions will not be aligned with the transmission of the other network devices. This misalignment may result in network degradation, as the misaligned network device may cause, when transmitting, interference to other network devices. Also, the misaligned network device may be subjected to interference when other network devices are transmitting. In 4th Generation (4G)/long term evolution (LTE) wireless communication system, for example, the 3GPP TS36.133 standard requires that time synchronization of network devices of a TDD network be within 3 us.
Several techniques exist for synchronizing the network devices. For example, synchronization of network devices may be accomplished by providing the network devices with synchronization signals, which the network devices can use for performing the synchronization. Such synchronization signals may be provided to the network devices from a common synchronization source (hereinafter referred to as “global synchronization source”).
Under good backhaul conditions, for example, with operator controlled fiber or Ethernet, synchronization signals provided in accordance with IEEE 1588 v2 can provide sub-microsecond level accuracy. However, such good backhaul conditions may not always be possible. In particular, backhauls over cable and digital subscriber line (DSL) modems have significant jitter and delay variations. Moreover, IEEE 1588 server or switch support must be provided within the network in order to provide the requisite backhaul.
In another technique in which synchronization signals are utilized for synchronizing the network devices, network devices (e.g., enhanced Node-B (eNB), access point, mobile device, etc.) that include a global positioning system (GPS) receiver and can acquire the GPS synchronization signals. However, GPS receivers often only reliably receive GPS reference signals when the device is provided with a relatively unobstructed view of the sky (e.g., disposed outdoors, in an area free of shadowing from terrain, foliage, and structures, etc.), and thus often does not provide a reliable synchronization technique for many network devices (e.g., home eNB (HeNB), femtocell, etc.).
Network listening provides a technique in which synchronization signals are utilized for synchronizing the network devices that can be used in scenarios where GPS and IEEE 1588 v2 do not work. In situations where the network listening technique may be advantageous, a particular network device may not be able to receive, or reliably receive, synchronization signals from the global synchronization source (e.g., GPS reference signals). For example, network devices disposed indoors, in an area shadowed by tall buildings or terrain, etc., may be unable to reliably receive GPS reference signals for establishing synchronization with other network devices. A network device that is unable to reliably receive synchronization signals from a global synchronization source may, nevertheless, be within communications range of other network devices, wherein one or more such network devices may be capable of receiving synchronization signals from the global synchronization source, and thus may employ network listening techniques to obtain relayed or retransmitted synchronization signals. For example, network devices that are able to receive, or reliably receive, synchronization signals from the global synchronization source may transmit (e.g., broadcast) synchronization signals for use by the network devices that are unable to receive, or reliably receive, the synchronization signals from the global synchronization source. Accordingly, the network devices that are unable to reliably receive synchronization signals from the global synchronization source are able to nevertheless synchronize their communications to the other network devices by using a synchronization signal broadcast by another network device (the particular network device broadcasting the synchronization signal selected/used by another network device being referred to herein as a “synchronization target” and a “parent network device” for the network device receiving the broadcast synchronization signal and the network device using the broadcast synchronization signal being referred to herein as a “child network device”). One of the advantages of the network listening technique is that no extra requirement is required on backhaul and it can be applied in both out-door and in-door conditions.
It should be appreciated that the relaying of synchronization signals in accordance with a network listening technique can be used for any number of network devices that cannot reliably receive synchronization signals from the global synchronization source but are within communications range of other network devices that are able to reliably receive synchronization signals from the global synchronization source. Moreover, child network devices reliably receiving synchronization signals transmitted from a parent network device, and thus establishing network synchronization, may themselves transmit (e.g., broadcast or rebroadcast) synchronization signals such that other network devices (child network devices of higher (worse) stratum level of a synchronization tree structure) that are within communications range of the first mentioned child network devices are able to receive synchronization signals. In this case, the higher (worse) stratum level child network devices receiving these synchronization signals can use the lower (better) stratum level child network devices transmitting synchronization signals as synchronization targets and the child network devices transmitting these synchronization signals are themselves parent network devices to the higher (worse) stratum level child network devices receiving the synchronization signals. Accordingly, although various network devices may not receive the synchronization signals directly from the global synchronization source, all of the network devices may nonetheless be synchronized to the other devices in the network via their respective synchronization target, either by synchronizing to the global synchronization source using synchronization signals transmitted by a network device receiving the synchronization signals directly from the global synchronization source or using synchronization signals relayed by one or more network devices.
As can be seen from the foregoing, in some instances of a time synchronized network, a synchronization target utilized by any particular child network device may itself not be receiving synchronization signals directly from the global synchronization source, but instead may be receiving synchronization signals transmitted by an intermediate, intervening child network device serving as a synchronization target. This interconnection of the various network devices to each other as parent and child network devices results in a synchronization target hierarchy, in the form of a synchronization tree with the various network devices as network devices of the tree, for providing synchronization signals to the variously disposed network devices. Network devices in such a synchronization tree may be children network devices of their respective synchronization target parent network devices, and synchronization target parent network devices may be parent network devices to their respective children network devices. Furthermore, a network device may have a child network device that itself has one or more child network devices. Such child network devices (i.e., the initial child network device and any child network devices thereof) are said to be downstream of the parent network device, and are said to be downstream children of the parent network device.
In accordance with the aforementioned synchronization tree structure, some network devices may be directly receiving synchronization signals from the global synchronization source and thus, may be said to be one hop away from the global synchronization source. These network devices are thus said to have a synchronization tree stratum level of one. Network devices receiving synchronization signals transmitted from a synchronization target in the synchronization tree are said to be more than one hop away from the global synchronization source due to the synchronization signal being provided by one or more intermediary, intervening network devices, wherein the number of hops, and correspondingly their synchronization tree stratum level, depends upon the network device's placement in the synchronization tree hierarchy. Accordingly, a stratum level of such a network device indicates the number of hops that exist between the network device and the global synchronization source of the network.
A stratum index of a network device may be used to indicate the particular stratum level of the network device. Thus, network devices that are receiving synchronization signals directly from the global synchronization source may be said to have a stratum level of 1, and may be configured with a stratum index of 1. A child network device using a stratum level 1 network device as synchronization target may be said to be two hops away from the global synchronization source, i.e., one hop from the device to the synchronization target, and another hop from the synchronization target to the global synchronization source. Such a child network device may thus be configured with a stratum index of 2. A further child network device using a stratum level 2 child network device as synchronization target may be said to be three hops away from the global synchronization source, and may be configured with a stratum index of 3, and so on. Generally, the stratum index of a network device may be equal to the stratum index of its synchronization target plus a non-zero, positive number. The non-zero, positive number may indicate a distance between the network device and its synchronization target. For example, the stratum index of a network device may be equal to the stratum index of its synchronization target plus the distance, in hops, between the network device and its synchronization target.
In operation of a wireless network, any particular network device may fail. However, it is often difficult to maintain global synchronization of the network devices when some nodes fail due to the synchronization tree structure of a network listening technique. Any child network device configured with the failed network device as its synchronization target, as well as any further child network devices downstream in the synchronization tree, will be in danger of losing synchronization. Thus, fast and efficient selection of a new synchronization target for the network device is of paramount importance. A number of techniques for selecting a new synchronization target have been utilized. However, these techniques have generally not been well suited for use with respect to some network scenarios and/or configurations. For example, the existing synchronization techniques are overly restrictive, and often ineffective, when operating under certain standard operational constraints.